Thermoelectric materials

ABSTRACT

Disclosed is a thermoelectric conversion material having excellent performance. The thermoelectric material according to the present disclosure includes Cu and Se, and has a plurality of different crystal structures together, in which Cu atoms and Se atoms are arranged in the crystal, at a predetermined temperature.

TECHNICAL FIELD

The present disclosure relates to thermoelectric conversion technology,and more particularly, to a thermoelectric conversion material withexcellent thermoelectric conversion properties and its manufacturingmethod.

The present application claims priority to Korean Patent Application No.10-2013-0107927 filed on Sep. 9, 2013 in the Republic of Korea, KoreanPatent Application No. 10-2014-0091973 filed on Jul. 21, 2014 in theRepublic of Korea, and Korean Patent Application No. 10-2014-0117863filed on Sep. 4, 2014 in the Republic of Korea, the disclosures of whichare incorporated herein by reference.

BACKGROUND ART

A compound semiconductor is a compound that is composed of at least twotypes of elements rather than one type of element such as silicon orgermanium and operates as a semiconductor. Various types of compoundsemiconductors have been developed and are currently being used invarious fields of industry. Typically, a compound semiconductor may beused in thermoelectric conversion elements using the Peltier Effect,light emitting devices using the photoelectric conversion effect, forexample, light emitting diodes or laser diodes, fuel cells, and thelike.

Particularly, a thermoelectric conversion element is used forthermoelectric conversion power generation or thermoelectric conversioncooling applications, and generally includes an N-type thermoelectricsemiconductor and a P-type thermoelectric semiconductor electricallyconnected in series and thermally connected in parallel. Thethermoelectric conversion power generation is a method which generatespower by converting thermal energy to electrical energy using athermoelectromotive force generated by creating a temperature differencein a thermoelectric conversion element. Also, the thermoelectricconversion cooling is a method which produces cooling by convertingelectrical energy to thermal energy using an effect that a temperaturedifference creates between both ends of a thermoelectric conversionelement when a direct current flows through the both ends of thethermoelectric conversion element.

The energy conversion efficiency of the thermoelectric conversionelement generally depends on a performance index value or ZT of athermoelectric conversion material. Here, the ZT may be determined basedon the Seebeck coefficient, electrical conductivity, and thermalconductivity, and as a ZT value increases, a thermoelectric conversionmaterial has better performance.

Many thermoelectric materials available for a thermoelectric conversionelement have been now proposed and developed, and among them, Cu_(x)Se(x≦2) was proposed as a Cu—Se based thermoelectric material and is beingdeveloped. This is because Cu_(x)Se (x≦2) is known.

Particularly, it has been recently reported that a relatively lowthermal conductivity and a high ZT value was achieved in Cu_(x)Se(1.98≦x≦2). Typically, Lidong Chen group has reported that Cu₂Seexhibited ZT=1.5 at 727° C. (Nature Materials, 11, (2012), 422-425).Also, Gang Chen group of MIT has reported a high ZT value atx=1.96(Cu₂Se_(1.02)) and x=1.98(Cu₂Se_(1.01)) (x is less than 2) (NanoEnergy (2012) 1, 472-478).

However, seeing both of the two results, a comparatively good ZT valuewas observed at 600° C.˜727° C., but a ZT value was found very low atthe temperature lower than or equal to 600° C. Even though athermoelectric material has a high ZT at a high temperature, if thethermoelectric material has a low ZT value at a low temperature, such athermoelectric material is not preferred, in particular, unsuitable fora thermoelectric material for power generation. Even if such athermoelectric material is applied to a heat source of high temperature,a certain region of the material is subjected to a temperature muchlower than a desired temperature due to a temperature gradient in thematerial itself Therefore, there is a need to develop a thermoelectricmaterial capable of maintaining a high ZT value over a broad temperaturerange due to having a high ZT value in a temperature range lower than orequal to 600° C., for example, 100° C.˜600° C., as well as in atemperature range higher than 600° C.

DISCLOSURE Technical Problem

Accordingly, the present disclosure is designed to solve the aboveproblem, and therefore, the present disclosure is directed to providinga thermoelectric material having high thermoelectric conversionperformance over a broad temperature range and its manufacturing method.

These and other objects and advantages of the present disclosure may beunderstood from the following detailed description and will become morefully apparent from the exemplary embodiments of the present disclosure.Also, it will be easily understood that the objects and advantages ofthe present disclosure may be realized by the means shown in theappended claims and combinations thereof.

Technical Solution

To achieve the above object, a thermoelectric material according to thepresent disclosure includes Cu and Se, and has a plurality of differentcrystal structures together, in which Cu atoms and Se atoms are arrangedin the crystal, at a predetermined temperature.

Preferably, the thermoelectric material according to the presentdisclosure may have a plurality of different crystal structures at apredetermined temperature within a temperature range of 100° C. to 300°C.

Also, preferably, the thermoelectric material according to the presentdisclosure may have a monoclinic crystal structure and a cubic crystalstructure together at 100° C.

Also, preferably, the thermoelectric material according to the presentdisclosure may have one type of monoclinic crystal structure and twotypes of cubic crystal structures together at 100° C.

Also, preferably, the thermoelectric material according to the presentdisclosure may have two different cubic crystal structures of differentspace groups together at one or more temperatures of 150° C., 200° C.,and 250° C.

Also, preferably, in the thermoelectric material according to thepresent disclosure, the different space groups of the cubic crystalstructures may be indicated by Fm-3m and F-43m, respectively.

Also, preferably, the thermoelectric material according to the presentdisclosure may be represented by the following chemical formula 1:Cu_(x)Se   <Chemical Formula 1>

where x is a positive rational number.

Also, preferably, in the chemical formula 1, 2≦x≦2.6. Also, to achievethe above object, a thermoelectric conversion element according to thepresent disclosure includes the thermoelectric material according to thepresent disclosure.

Also, to achieve the above object, a thermoelectric power generatoraccording to the present disclosure includes the thermoelectric materialaccording to the present disclosure.

Advantageous Effects

According to the present disclosure, a thermoelectric material havingexcellent thermoelectric conversion performance may be provided.

Particularly, the thermoelectric material according to one aspect of thepresent disclosure may have a low thermal diffusivity and a low thermalconductivity and a high Seebeck coefficient and a high ZT value in abroad temperature range between 100° C. and 600° C.

Accordingly, the thermoelectric material according to the presentdisclosure may replace a traditional thermoelectric material, or may beused as another material in conjunction with a traditionalthermoelectric material.

Moreover, the thermoelectric material according to the presentdisclosure may maintain a ZT value higher than a traditionalthermoelectric material at a temperature lower than or equal to 600° C.,to be more concrete, at a low temperature close to 100° C.˜200° C. Thus,when used in a thermoelectric device for power generation, thethermoelectric material according to the present disclosure may ensurestable thermoelectric conversion performance even if the material isexposed to a comparatively low temperature.

Also, the thermoelectric material according to the present disclosuremay be used in a solar cell, an infrared (IR) window, an IR sensor, amagnetic device, a memory, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate a preferred embodiment of thepresent disclosure and together with the foregoing disclosure, serve toprovide further understanding of the technical spirit of the presentdisclosure, and thus, the present disclosure is not construed as beinglimited to the drawing.

FIG. 1 is a graph of an X-ray diffraction (XRD) analysis result based ontemperature for a thermoelectric material according to an exemplaryembodiment of the present disclosure.

FIG. 2 is a graph of an XRD analysis result of a thermoelectric materialaccording to exemplary embodiments of the present disclosure.

FIG. 3 is an enlarged graph of section A of FIG. 2.

FIGS. 4 through 8 are diagrams illustrating a scanning electronmicroscope/energy dispersive spectroscopy (SEM/EDS) analysis result of athermoelectric material according to an exemplary embodiment of thepresent disclosure.

FIG. 9 is a flow chart schematically illustrating a method formanufacturing a thermoelectric material according to an exemplaryembodiment of the present disclosure.

FIG. 10 is a graph illustrating a comparison of thermal diffusivitymeasurement results based on temperature for thermoelectric materialsaccording to examples of the present disclosure and comparativeexamples.

FIG. 11 is a graph illustrating a comparison of Seebeck coefficientmeasurement results based on temperature for thermoelectric materialsaccording to examples of the present disclosure and comparativeexamples.

FIG. 12 is a graph illustrating a comparison of ZT value measurementresults based on temperature for thermoelectric materials according toexamples of the present disclosure and comparative examples.

FIG. 13 is a scanning ion microscope (SIM) image of a thermoelectricmaterial according to an example of the present disclosure.

FIG. 14 is an SIM image of a thermoelectric material according to acomparative example.

FIG. 15 is a graph with a change in y-axis scale only for the examplesof FIG. 10.

FIG. 16 is a graph with a change in y-axis scale only for the examplesof FIG. 11.

FIG. 17 is a graph illustrating a comparison of lattice thermalconductivity measurement results based on temperature for thermoelectricmaterials according to different exemplary embodiments of the presentdisclosure, manufactured by different synthesis methods.

FIG. 18 is a graph illustrating a comparison of power factor measurementresults based on temperature for thermoelectric materials according todifferent exemplary embodiments of the present disclosure, manufacturedby different synthesis methods.

FIG. 19 is a graph illustrating a comparison of ZT value measurementresults based on temperature for thermoelectric materials according todifferent exemplary embodiments of the present disclosure, manufacturedby different synthesis methods.

DETAILED DESCRIPTION

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings. Priorto the description, it should be understood that the terms used in thespecification and the appended claims should not be construed as limitedto general and dictionary meanings, but interpreted based on themeanings and concepts corresponding to technical aspects of the presentdisclosure on the basis of the principle that the inventor is allowed todefine terms appropriately for the best explanation.

Therefore, the description proposed herein is just a preferable examplefor the purpose of illustrations only, not intended to limit the scopeof the disclosure, so it should be understood that other equivalents andmodifications could be made thereto without departing from the spiritand scope of the disclosure.

A thermoelectric material according to the present disclosurecorresponds to a thermoelectric material including Cu and Se, and havinga plurality of crystal structures at a predetermined temperature. Thatis, the thermoelectric material according to the present disclosure mayexist, at a predetermined temperature, in a form of at least two typesof crystal structures in which Cu atoms and Se atoms are arranged in thecrystal.

Particularly, the thermoelectric material according to the presentdisclosure may have a plurality of different crystal lattice structuresat a predetermined temperature within the temperature range of 100° C.to 300° C.

FIG. 1 is a graph of an X-ray diffraction (XRD) analysis result based ontemperature for a thermoelectric material according to an exemplaryembodiment of the present disclosure.

More specifically, FIG. 1 is an XRD graph measured in the respectivetemperature conditions of 25° C., 50° C., 100° C., 150° C., 200° C.,250° C., 300° C., and 350° C., for a thermoelectric material representedby a chemical formula Cu_(2.1)Se in terms of composition as an exampleof the present disclosure.

Also, in FIG. 1, a typical representation is provided for each peakcorresponding to four phases of Cubic_Fm-3m, Cubic_F-43m,Monoclinic_C2/C, and Cu_Cubic_Fm-3m. For example, in FIG. 1, a peakcorresponding to Cubic_Fm-3m crystal structure is indicated by a squaresymbol, a peak corresponding to Cubic_F-43m crystal structure isindicated by an inverse triangle symbol, a peak corresponding toMonoclinic_C2/C crystal structure is indicated by a star symbol, and apeak corresponding to Cu_Cubic_Fm-3m crystal structure is indicated by atrapezoid symbol.

Referring to FIG. 1, at 25° C. and 50° C., except a peak correspondingto a cubic structure (Cu_Cubic_Fm-3m) by Cu particles existingsingularly, it can be seen that only a peak corresponding to monocliniccrystal structure (Monoclinic_C2/C) primarily occurs. Thus, in the caseof the thermoelectric material according to the present disclosure, itcan be seen that a crystal composed of Cu atoms and Se atoms is in asingle phase of monoclinic structure (Monoclinic_C2/C) at thetemperature lower than or equal to 50° C.

However, seeing the measurement result at 100° C., a peak correspondingto a cubic crystal structure is found together with a peak correspondingto a monoclinic crystal structure. That is, at 100° C., a monocliniccrystal structure is dominant, but a cubic crystal structure may befound. Thus, the thermoelectric material according to the presentdisclosure may have a plurality of crystal structures including both amonoclinic crystal structure and a cubic crystal structure at the sametime in the temperature condition of 100° C.

Moreover, in the embodiment of FIG. 1, as a peak corresponding to acubic crystal structure, peaks for two cubic crystal structures (CubicFm-3m, Cubic_F-43m) of different space groups are all observed. Thus,the thermoelectric material according to the present disclosure may havea crystal structure including one type of monoclinic crystal structure(Monoclinic_C2/C) and two types of cubic crystal structures in thetemperature condition of 100° C. In this case, the thermoelectricmaterial according to this aspect of the present disclosure may have, inthe temperature condition of 100° C., at least three crystal structuresin which Cu atoms and Se atoms are arranged in the crystal.

Also, when the temperature increases from 50° C. to 100° C., thethermoelectric material according to the present disclosure may be proneto a phase transition from a monoclinic crystal structure to two typesof cubic crystal structures in some parts of the monoclinic crystalstructure.

Also, seeing the measurement result at 150° C., 200° C., and 250° C., itcan be seen that a peak corresponding to a monoclinic phase nearlydisappears and only peaks corresponding to two cubic phases areprimarily found. Thus, the thermoelectric material according to thepresent disclosure may have two types of cubic crystal structures(Cubic_Fm-3m, Cubic_F-43m) of different space groups, in which Cu atomsand Se atoms are arranged in the crystal, in the temperature conditionof 150° C. to 250° C., in particular, in at least one temperaturecondition of 150° C., 200° C., and 250° C. Also, in this instance, thespace groups of the two types of cubic crystal structures may beindicated by Fm-3m and F-43m, respectively.

Accordingly, the thermoelectric material according to the presentdisclosure may be prone to a phase transition from a monoclinic crystalstructure to a cubic crystal structure in a great part of the monocliniccrystal structure with the increasing temperature from 100° C. to 150°C.

Further, referring to the measurement result of FIG. 1, thethermoelectric material according to the present disclosure may have arelative increase in ratio of F-43m cubic crystal structure with theincreasing temperature from 150° C. to 200° C.

Also, referring to the measurement result of FIG. 1, the thermoelectricmaterial according to the present disclosure may have a relativedecrease in ratio of F-43m cubic crystal structure with the increasingtemperature from 200° C. to 250° C.

Meanwhile, from the measurement result of FIG. 1, it can be seen thatonly a peak corresponding to a Cubic_Fm-3m phase primarily occurs at300° C. and 350° C. Thus, it can be seen that the thermoelectricmaterial according to the present disclosure is in a form of a singlecrystal structure of Cubic_Fm-3m at the temperature higher than or equalto 300° C. Also, according to the result, it can be seen that thethermoelectric material according to the present disclosure is just in asingle phase of Fm-3m cubic crystal structure while an F-43m cubiccrystal structure disappears, with the increasing temperature from 250°C. to 300° C.

As noted in the XRD measurement result, the thermoelectric materialaccording to the present disclosure may have a plurality of differentcrystal structures together, in which Cu atoms and Se atoms are arrangedin the crystal, in a predetermined temperature condition within thetemperature range of 100° C. to 300° C.

Preferably, the thermoelectric material according to one aspect of thepresent disclosure may be represented by the following chemical formula1 in terms of composition:Cu_(x)Se   <Chemical Formula 1>

where x is a positive rational number.

Preferably, in the chemical formula 1, 2≦x≦2.6.

More preferably, in the chemical formula 1, the condition of x<2.2 maybe satisfied. Particularly, in the chemical formula 1, x≦2.2.

More preferably, in the chemical formula 1, the condition of x≦2.15 maybe satisfied.

Particularly, in the chemical formula 1, the condition of x≦2.1 may besatisfied.

Also, preferably, in the chemical formula 1, the condition of 2.01≦x maybe satisfied. Particularly, in the chemical formula 1, 2.01≦x.

More preferably, in the chemical formula 1, the condition of 2.025≦x maybe satisfied. Under these conditions, the thermoelectric conversionperformance of the thermoelectric material according to the presentdisclosure may be further improved

Particularly, in the chemical formula 1, the condition of 2.04≦x may besatisfied.

Preferably, in the chemical formula 1, the condition of 2.05≦x may besatisfied.

More preferably, in the chemical formula 1, the condition of 2.075≦x maybe satisfied.

Meanwhile, a second phase may be included in the thermoelectric materialrepresented by the chemical formula 1 in part, and its amount may changebased on the heat treatment condition.

Also, the thermoelectric material according to one aspect of the presentdisclosure includes a Cu—Se matrix including Cu and Se, andCu-containing particles. Here, the Cu-containing particles representparticles containing at least Cu, and may include particles containingonly Cu and particles containing Cu and at least one element other thanCu.

Preferably, the Cu-containing particles may include at least one of Cuparticles having a single Cu composition and Cu oxide particles havingCu—O bonds such as, for example, Cu₂O particles.

Particularly, the thermoelectric material according to the presentdisclosure may include induced nano-dots (INDOT) as the Cu-containingparticles. Here, the INDOT may represent particles of a nanometer size(for example, a size of 1 nanometer to 100 nanometers in diameter)spontaneously generated during production of the thermoelectric materialaccording to the present disclosure. That is, in the present disclosure,the INDOT may be particles induced by itself within the thermoelectricmaterial during production of the thermoelectric material, rather thanparticles artificially introduced into the thermoelectric material fromoutside.

Further, in the present disclosure, the nano-dots, or INDOT may bepresent at a grain boundary of a semiconductor. Also, in the presentdisclosure, the INDOT may be generated at the grain boundary duringproduction of the thermoelectric material, particularly, duringsintering. In this instance, the INDOT included in the thermoelectricmaterial according to the present disclosure may be defined as nano-dotsspontaneously induced at the grain boundary of the semiconductor duringsintering (induced nano-dots on grain boundary). In this case, thethermoelectric material according to the present disclosure may includethe Cu—Se matrix and the INDOT.

Based on the chemical formula in terms of composition, thethermoelectric material according to the present disclosure may includea larger amount of Cu than a traditional Cu—Se based thermoelectricmaterial. In this instance, at least a part of the Cu does not form amatrix with Se, and may exist singularly as a single element or incombination with other element, for example, oxygen, and Cu existingsingularly or in combination with other element may be included in aform of nano-dots. Its detailed description is provided with referenceto experiment results.

FIG. 2 is a graph of an XRD analysis result of a thermoelectric materialaccording to exemplary embodiments of the present disclosure, and FIG. 3is an enlarged graph of section A of FIG. 2.

More specifically, FIGS. 2 and 3 show a graph (x-axis units: degree) ofXRD pattern analysis of a thermoelectric material (manufactured by thesame method as the following examples 2˜5) including elements Cu and Seas represented by Cu_(x)Se (x=2.025, 2.05, 2.075, 2.1) in terms ofcomposition and having a plurality of different crystal structurestogether, in which Cu atoms and Se atoms are arranged in the crystal, ata predetermined temperature, as an example of the present disclosure.Particularly, for ease of distinguishment, in FIG. 2, the XRD patternanalysis graphs for each example are spaced a predetermined distanceaway from each other in the vertical direction. Also, for convenience ofcomparison, in FIG. 3, the graphs of each example are not spaced awayfrom each other and overlap with each other. Further, in FIG. 2, a Cupeak occurring at a single Cu composition is indicated by B.

Referring to FIGS. 2 and 3, it can be seen that as a relative content ofcopper in Cu_(x)Se, or x, gradually increases from 2.025 to 2.05, 2.075,and 2.1, a height of a Cu peak gradually increases. Thus, according tothe XRD analysis result, it can be found that as x gradually increases,more than 2, Cu in excess does not form a matrix such as Cu_(x)Se withSe and exists singularly.

Meanwhile, Cu existing without forming a matrix with Se may be in a formof nano-dots. Also, the Cu-containing nano-dots may exist in the way ofaggregating with each other within the thermoelectric material,particularly, at the grain boundary of the Cu—Se matrix.

FIGS. 4 through 8 are diagrams illustrating a scanning electronmicroscope/energy dispersive spectroscopy (SEM/EDS) analysis result of athermoelectric material according to an exemplary embodiment of thepresent disclosure.

More specifically, FIG. 4 is an SEM image of a part of a thermoelectricmaterial represented by a chemical formula Cu_(2.075)Se and having aplurality of different crystal structures together, in which Cu atomsand Se atoms are arranged in the crystal, at a predeterminedtemperature, as an example of the present disclosure. Also, FIGS. 5 and6 are SEM images of different parts of a thermoelectric materialrepresented by a chemical formula Cu_(2.1) Se and having a plurality ofdifferent crystal structures together, in which Cu atoms and Se atomsare arranged in the crystal, at a predetermined temperature, as anotherexample of the present disclosure. Also, FIG. 7 is a graph illustratingan EDS analysis result of section C1 of FIG. 4, and FIG. 8 is a graphillustrating an EDS analysis result of section C2 of FIG. 4.

First, referring to the images of FIGS. 3 through 6, it can be seen thatthere are a plurality of grains having a size from about severalmicrometers to tens of micrometers and a plurality of nano-dots having ananometer size smaller than the grains. In this instance, it can be seenthat the nano-dots may be formed along a grain boundary in a matrixincluding the plurality of grains as shown in the drawings, and at leastsome of the nano-dots may exist in the way of aggregating with eachother as indicated by C2. Particularly, referring to the SEM images ofFIGS. 5 and 6, it can be apparently seen that the nano-dots aredistributed in large amounts along the grain boundary in the Cu—Sematrix.

Next, referring to FIG. 7 illustrating an analysis result of section C1of FIG. 4 where no nano-dot is observed, that is, internal analysis ofthe grain, it can be seen that a Cu peak and a Se peak primarily occur.From this, it can be found that Cu and Se form a matrix in section C1 ofFIG. 4. That is, the grains shown in FIG. 4 may be a Cu—Se grain havingCu and Se as a main component. Also, through a quantitative analysis,the Cu—Se matrix may exist as Cu_(x)Se in which x has 2 or a value closeto 2.

In contrast, referring to FIG. 8 illustrating an analysis result ofsection C2 of FIG. 4 where aggregation of nano-dots is observed, it canbe seen that a Cu peak is formed dominantly high. It can be found thatthe nano-dots exist as Cu rather than a Cu—Se matrix. The reason that aSe peak is observed a little bit is because Se found in the Cu—Se matrixlocated around or under the nano-dots is measured due to the limit ofthe resolving power of analysis equipment or the limit of an analysismethod.

Accordingly, based on these results, it can be found that the particlesconcentrated on section C2 of FIG. 4 are Cu-containing nano-dots. Thus,the thermoelectric material according to one aspect of the presentdisclosure may be a thermoelectric material including Cu particles,particularly, Cu-containing INDOT, together with the Cu—Se matrixincluding Cu and Se. Particularly, at least a part of the Cu-containingINDOT may exist in the way of aggregating with each other in thethermoelectric material. Here, the Cu-containing INDOT may include Cusingularly, but as shown in FIG. 8 illustrating that an O peak isobserved a little bit, the Cu-containing INDOT may exist in a form of Cuoxide having bonds with O, for example, Cu₂O.

As described in the foregoing, the thermoelectric material according toone aspect of the present disclosure may include Cu-containingnano-dots, particularly, INDOT and a Cu—Se matrix. Here, the Cu—Sematrix may be represented by a chemical formula Cu_(x)Se in which x is apositive rational number. Particularly, x may have a value near 2, forexample, 1.8˜2.2. Further, x may have a value less than or equal to 2,for example, 1.8˜2.0. For example, the thermoelectric material accordingto the present disclosure may include a Cu₂Se matrix and Cu-containingnano-dots.

Here, the Cu-containing nano-dots may be present at the grain boundaryin the Cu—Se matrix as previously described. For example, thethermoelectric material according to the present disclosure may includea Cu₂Se matrix and copper particles of a single composition at the grainboundary in the Cu₂Se matrix. It is obvious that some of theCu-containing nano-dots may be present within the grains in the Cu—Sematrix.

Meanwhile, the thermoelectric material according to one aspect of thepresent disclosure corresponds to a Cu—Se based thermoelectric materialincluding Cu and Se, and has a lower thermal conductivity and a higherZT value than a traditional Cu—Se based thermoelectric material.

Particularly, the thermoelectric material according to the presentdisclosure may include Cu and Se, and have a plurality of differentcrystal structures together, in which Cu atoms and Se atoms are arrangedin the crystal, at a predetermined temperature.

The thermoelectric material according to the present disclosure may havea thermal diffusivity less than or equal to 0.5 mm²/s in the temperaturerange of 100° C. to 600° C.

Also, the thermoelectric material according to the present disclosuremay have a ZT value higher than or equal to 0.3 over the entiretemperature range 100° C. to 600° C.

Particularly, the thermoelectric material according to the presentdisclosure may have a ZT value higher than or equal to 0.3 in thetemperature condition of 100° C. Preferably, the thermoelectric materialaccording to the present disclosure may have a ZT value higher than orequal to 0.4 in the temperature condition of 100° C.

Also, the thermoelectric material according to the present disclosuremay have a ZT value higher than or equal to 0.4 in the temperaturecondition of 200° C. Preferably, the thermoelectric material accordingto the present disclosure may have a ZT value higher than or equal to0.5 in the temperature condition of 200° C. More preferably, thethermoelectric material according to the present disclosure may have aZT value higher than 0.6 in the temperature condition of 200° C.

Also, the thermoelectric material according to the present disclosuremay have a ZT value higher than or equal to 0.6 in the temperaturecondition of 300° C. Preferably, the thermoelectric material accordingto the present disclosure may have a ZT value higher than or equal to0.75 in the temperature condition of 300° C. More preferably, thethermoelectric material according to the present disclosure may have aZT value higher than 0.8 in the temperature condition of 300° C. Morepreferably, the thermoelectric material according to the presentdisclosure may have a ZT value higher than 0.9 in the temperaturecondition of 300° C.

Also, the thermoelectric material according to the present disclosuremay have a ZT value higher than or equal to 0.7 in the temperaturecondition of 400° C. Preferably, the thermoelectric material accordingto the present disclosure may have a ZT value higher than or equal to0.8 in the temperature condition of 400° C. More preferably, thethermoelectric material according to the present disclosure may have aZT value higher than or equal to 1.0 in the temperature condition of400° C.

Also, the thermoelectric material according to the present disclosuremay have a ZT value higher than or equal to 0.6 in the temperaturecondition of 500° C. Preferably, the thermoelectric material accordingto the present disclosure may have a ZT value higher than or equal to0.7 in the temperature condition of 500° C. More preferably, thethermoelectric material according to the present disclosure may have aZT value higher than or equal to 1.1 in the temperature condition of500° C. More preferably, the thermoelectric material according to thepresent disclosure may have a ZT value higher than or equal to 1.3 inthe temperature condition of 500° C.

Also, the thermoelectric material according to the present disclosuremay have a ZT value higher than or equal to 0.6 in the temperaturecondition of 600° C. Preferably, the thermoelectric material accordingto the present disclosure may have a ZT value higher than or equal to0.8 in the temperature condition of 600° C. More preferably, thethermoelectric material according to the present disclosure may have aZT value higher than or equal to 1.4 in the temperature condition of600° C. More preferably, the thermoelectric material according to thepresent disclosure may have a ZT value higher than or equal to 1.8 inthe temperature condition of 600° C.

The thermoelectric material according to the present disclosure may bemanufactured by the following method for manufacturing a thermoelectricmaterial.

FIG. 9 is a flow chart schematically illustrating a method formanufacturing a thermoelectric material according to an exemplaryembodiment of the present disclosure.

As shown in FIG. 9, the method for manufacturing a thermoelectricmaterial according to the present disclosure, including Cu and Se andhaving a plurality of different crystal structures together, in which Cuatoms and Se atoms are arranged in the crystal, at a predeterminedtemperature, includes a mixture forming step (S110) and a compoundforming step (S120).

The mixture forming step S110 is a step for mixing Cu and Se as a rawmaterial to form a mixture. Particularly, in S110, the mixture may beformed by weighing Cu and Se based on the chemical formula weight of theabove chemical formula 1, i.e., Cu_(x)Se (x is a positive rationalnumber) and mixing them.

Particularly, in S110, Cu and Se may be mixed in the range of 2≦x≦2.6 ofthe above chemical formula 1.

Preferably, in S110, Cu and Se in powder form may be mixed. In thiscase, Cu and Se may be mixed better, resulting in more favorablesynthesis of Cu_(x)Se.

In this instance, mixing of Cu and Se in the mixture forming step S110may be performed by hand milling using a mortar, ball milling, planetaryball mill, and the like, but the present disclosure is not limited tothese specific mixing methods.

The compound forming step S120 is a step for thermally treating themixture formed in S110 to form a Cu_(x)Se compound (x is a positiverational number, particularly, 2≦x≦2.6). For example, in S120, theCu_(x)Se compound may be formed by putting the mixture of Cu and Se intoa furnace and heating for a predetermined time at a predeterminedtemperature.

Preferably, S120 may be performed by a solid state reaction (SSR)method. When the synthesis is performed by the solid state reactionmethod, the raw material used in the synthesis, that is, the mixture maycause reaction in a solid state without changing to a liquid stateduring the synthesis.

For example, S120 may be performed in the temperature range of 200° C.to 650° C. for 1 to 24 hours. Because the temperature is in atemperature range lower than a melting point of Cu, when the heating isperformed in the temperature range, the Cu_(x)Se compound may be formedin which Cu does not melt. Particularly, S120 may be performed under thetemperature condition of 500° C. for 15 hours.

According to this embodiment, the thermoelectric material according tothe present disclosure having a plurality of different crystalstructures at the same time at a predetermined temperature, for example,from 100° C. to 300° C., may be manufactured more favorably.

In S120, to form the Cu_(x)Se compound, the mixture of Cu and Se may beput into a hard mold and formed into pellets, and the mixture in pelletform may be put into a fused silica tube and vacuum-sealed. Also, thevacuum-sealed first mixture may be put into the furnace and thermallytreated.

Preferably, the method for manufacturing a thermoelectric materialaccording to the present disclosure may further include sintering thecompound under pressure (S130) after the compound forming step S120.

Here, S130 is preferably performed by a hot press (HP) or spark plasmasintering (SPS) technique. The thermoelectric material according to thepresent disclosure may be easy to obtain a high sintering density and athermoelectric performance improvement effect, when sintered by thepressure sintering technique.

For example, the pressure sintering step may be performed under thepressure condition of 30MPa to 200MPa. Also, the pressure sintering stepmay be performed under the temperature condition of 300° C. to 800° C.Also, the pressure sintering step may be performed under the pressureand temperature conditions for 1 minute to 12 hours.

Also, S130 may be performed in a vacuum state, or while flowing gas suchas Ar, He, N₂, and the like, containing some or no hydrogen.

Also, preferably, S130 may be performed by grinding the compound formedin S120 into powder, and then performing pressure sintering. In thiscase, convenience in the sintering and measuring step may be improvedand the sintering density may further increase.

Particularly, the Cu-containing particles included in the thermoelectricmaterial according to the present disclosure may be spontaneously formedduring the pressure sintering step S130. That is, the Cu-containingparticles of the thermoelectric material according to the presentdisclosure are not forcibly introduced from outside, and may bespontaneously induced during the manufacturing process, particularly,during the sintering step. Accordingly, the Cu-containing particlesaccording to the present disclosure may be INDOT (Induced Nano DOT).According to this aspect of the present disclosure, the Cu-containingparticles may be easily formed without the need for intensive efforts tointroduce the Cu-containing particles into the thermoelectric material,particularly, at the grain boundary.

A thermoelectric conversion element according to the present disclosuremay include the above thermoelectric material. Particularly, thethermoelectric material according to the present disclosure mayeffectively improve a ZT value in a broad temperature range, compared toa traditional thermoelectric material, particularly, a Cu—Se basedthermoelectric material. Thus, the thermoelectric material according tothe present disclosure may replace a traditional thermoelectricconversion material or may be effectively used in a thermoelectricconversion element in conjunction with a traditional compoundsemiconductor.

Further, the thermoelectric material according to the present disclosuremay be used in a thermoelectric power generator designed forthermoelectric power generation using a waste heat source, etc. That is,the thermoelectric power generator according to the present disclosureincludes the above thermoelectric material according to the presentdisclosure. The thermoelectric material according to the presentdisclosure exhibits a high ZT value in a broad temperature range such asa temperature range of 100° C. to 600° C., and thus, may be applied tothermoelectric power generation more usefully.

Also, the thermoelectric material according to the present disclosuremay be manufactured as a bulk-type thermoelectric material.

Hereinafter, the present disclosure will be described in detail throughexamples and comparative examples. The examples of the presentdisclosure, however, may take several other forms, and the scope of thepresent disclosure should not be construed as being limited to thefollowing examples. The examples of the present disclosure are providedto more fully explain the present disclosure to those having ordinaryknowledge in the art to which the present disclosure pertains.

EXAMPLE 1

Cu and Se in powder form were weighed based on a chemical formulaCu_(2.01)Se, and put in an alumina mortar, followed by mixing. The mixedmaterials were put into a hard mold, formed into pellets, put in a fusedsilica tube, and vacuum-sealed. Also, the result was put in a boxfurnace, and heated at 500° C. for 15 hours, and after heating, wasslowly cooled down to room temperature to obtain a compound.

Also, the Cu_(2.01)Se compound was filled in a hard mold for hotpressing, and was hot press sintered in the condition of 650° C. undervacuum to obtain a sample of example 1. In this instance, a sinteringdensity was at least 98% of a theoretical value.

EXAMPLE 2

Cu and Se in powder form were weighed based on a chemical formulaCu_(2.025)Se, and mixed and synthesized by the same process as example 1to obtain a compound. Also, the compound was sintered by the sameprocess as example 1 to obtain a sample of example 2.

EXAMPLE 3

Cu and Se in powder form were weighed based on a chemical formulaCu_(2.05)Se, and mixed and synthesized by the same process as example 1to obtain a compound. Also, the compound was sintered by the sameprocess as example 1 to obtain a sample of example 3.

EXAMPLE 4

Cu and Se in powder form were weighed based on a chemical formulaCu_(2.075)Se, and mixed and synthesized by the same process as example 1to obtain a compound. Also, the compound was sintered by the sameprocess as example 1 to obtain a sample of example 4.

EXAMPLE 5

Cu and Se in powder form were weighed based on a chemical formulaCu_(2.1)Se, and mixed and synthesized by the same process as example 1to obtain a compound. Also, the compound was sintered by the sameprocess as example 1 to obtain a sample of example 5.

EXAMPLE 6

Cu and Se in powder form were weighed based on a chemical formulaCu_(2.15)Se, and mixed and synthesized by the same process as example 1to obtain a compound. Also, the compound was sintered by the sameprocess as example 1 to obtain a sample of example 6.

EXAMPLE 7

Cu and Se in powder form were weighed based on a chemical formulaCu_(2.2)Se, and mixed and synthesized by the same process as example 1to obtain a compound. Also, the compound was sintered by the sameprocess as example 1 to obtain a sample of example 7.

COMPARATIVE EXAMPLE 1

Cu and Se in powder form were weighed based on a chemical formulaCu_(1.8)Se, and mixed and synthesized by the same process as example 1to obtain a compound. Also, the compound was sintered by the sameprocess as example 1 to obtain a sample of comparative example 1.

COMPARATIVE EXAMPLE 2

Cu and Se in powder form were weighed based on a chemical formulaCu_(1.9)Se, and mixed and synthesized by the same process as example 1to obtain a compound. Also, the compound was sintered by the sameprocess as example 1 to obtain a sample of comparative example 2.

COMPARATIVE EXAMPLE 3

Cu and Se in powder form were weighed based on a chemical formulaCu_(2.0)Se, and mixed and synthesized by the same process as example 1to obtain a compound. Also, the compound was sintered by the sameprocess as example 1 to obtain a sample of comparative example 3.

It was found, as shown in FIG. 1, that the samples of examples 1˜7obtained in this way had a plurality of different crystal structures, inwhich Cu atoms and Se atoms are arranged in the crystal, at apredetermined temperature, particularly, at the temperature of 100° C.to 300° C. In contrast, it was found that the samples of comparativeexamples 1˜3 did not have a plurality of different crystal structures,in which Cu atoms and Se atoms are arranged in the crystal, at thepredetermined temperature.

For the samples of examples 1˜7 and the samples of comparative examples1˜3 obtained in this way, the thermal diffusivity (TD) was measured at apredetermined temperature interval using LFA457 (Netzsch), and itsresult is illustrated in FIG. 10 with examples 1˜7 and comparativeexamples 1˜3.

Also, for different parts of each of the samples of examples 1˜7 and thesamples of comparative examples 1˜3, the electrical conductivity andSeebeck coefficient of the samples were measured at a predeterminedtemperature interval using ZEM-3 (Ulvac-Riko, Inc), and its Seebeckcoefficient (S) measurement result is illustrated in FIG. 11 withexamples 1˜7 and comparative examples 1˜3. Also, a ZT value wascalculated using each of the measured values, and its result isillustrated in FIG. 12 with examples 1˜7 and comparative examples 1˜3.

First, referring to the result of FIG. 10, it can be seen that thethermoelectric materials according to examples 1˜7 having a plurality ofdifferent crystal structures at a predetermined temperature have aremarkably low thermal diffusivity over the entire temperaturemeasurement range of 100° C. to 700° C., in comparison to thethermoelectric materials according to comparative examples 1˜3 withouthaving a plurality of different crystal structures at a predeterminedtemperature.

Particularly, it can be seen that the samples of examples according tothe present disclosure have a thermal diffusivity lower than or equal to0.5 mm²/s, preferably, lower than 0.4 mm²/s, remarkably lower than thesamples of comparative examples, over the entire temperature range of100° C. to 600° C.

Next, referring to the result of FIG. 11, it can be seen that thethermoelectric materials of examples 1˜7 according to the presentdisclosure have a Seebeck coefficient much higher than thethermoelectric materials of comparative examples 1˜3 over the entiretemperature measurement range of 100° C. to 700° C.

Also, seeing ZT values of each sample with reference to the result ofFIG. 12, the thermoelectric materials of examples 1˜7 according to thepresent disclosure have a ZT value remarkably higher than thethermoelectric materials of comparative examples 1˜3.

Particularly, the thermoelectric materials according to comparativeexamples generally has a very low ZT value in the temperature rangelower than 500° C., and moreover, has a ZT value lower than or equal to0.2 in the low temperature range of 100° C. to 300° C.

In contrast, it can be seen that the thermoelectric materials accordingto examples of the present disclosure have a very high ZT value in thelow temperature range and the intermediate temperature range lower than500° C. as well as in the high temperature range higher than or equal to500° C., when compared to comparative examples.

In summary, the thermoelectric materials of examples 1˜6 showperformance improvement in ZT value about twice higher at 600° C. thanthe thermoelectric materials of comparative examples 1˜3.

More specifically, the thermoelectric materials of comparative examplesgenerally exhibit very low performance of a ZT value of 0.15 to 0.1 orlower in the temperature condition of 100° C., while the thermoelectricmaterials of examples according to the present disclosure exhibit highperformance of 0.3 to 0.4 or higher in the temperature condition of 100°C.

Also, in the temperature condition of 200° C., the thermoelectricmaterials of comparative examples exhibit a very low ZT value of 0.15 to0.1 or lower similar to the case of 100° C., while the thermoelectricmaterials of examples according to the present disclosure exhibit a highZT value of 0.4 or higher, to the maximum, 0.5˜0.7.

Also, in the temperature condition of 300° C., the thermoelectricmaterials of comparative examples exhibit a ZT value near about 0.1˜0.2,while the thermoelectric materials of examples according to the presentdisclosure all exhibit a value of 0.6 or higher, to the maximum, 0.7˜0.8or higher, with a large difference therebetween.

Also, in the temperature condition of 400° C., the thermoelectricmaterials of comparative examples exhibit a ZT value of 0.1˜0.2, to themaximum, about 0.35, while the thermoelectric materials of examplesaccording to the present disclosure all exhibit a value higher than orequal to 0.7, and most of them exhibit a high value of 0.8, to themaximum, 1.0˜1.2.

Also, in the temperature condition of 500° C., it can be seen that thethermoelectric materials of comparative examples exhibit a value lowerthan or equal to about 0.5, while the thermoelectric materials ofexamples according to the present disclosure exhibit a very high ZT of0.6 or higher, to the maximum, 1.0˜1.4.

Also, in the temperature condition of 600° C., the thermoelectricmaterials of comparative examples 1˜3 generally exhibit a ZT value of0.4-0.9, while the thermoelectric materials of examples 1˜5 according tothe present disclosure exhibit a very high ZT value of 1.4˜1.7, with alarge difference from the thermoelectric materials of comparativeexamples.

Taking the foregoing results into comprehensive consideration, it can beseen that the thermoelectric materials according to each example of thepresent disclosure having at least two types of crystal structures, inwhich Cu atoms and Se atoms are arranged in the crystal, at apredetermined temperature have a remarkably low thermal diffusivity anda remarkably high ZT value over the entire temperature range of 100° C.to 600° C., compared to the conventional thermoelectric materialsaccording to comparative examples. Accordingly, the thermoelectricmaterial according to the present disclosure is excellent inthermoelectric conversion performance, and may be used as athermoelectric conversion material very usefully.

Meanwhile, as described in the foregoing, the thermoelectric materialaccording to the present disclosure may further include Cu-containingnano-dots as well as the Cu—Se matrix. Its detailed description isprovided with reference to FIGS. 13 and 14.

FIG. 13 is a scanning ion microscope (SIM) image of the samplemanufactured in example 4, and FIG. 14 is an SIM image of the samplemanufactured in comparative example 3.

First, referring to FIG. 13, in the case of the thermoelectric materialaccording to example 4 of the present disclosure, nano-dots are found.Also, the nano-dots are Cu-containing nano-dots as previously noted.Particularly, as shown in FIG. 13, the nano-dots may be primarilydistributed along a grain boundary.

In contrast, referring to FIG. 14, it can be seen that a nano-dot isabsent in the traditional Cu—Se thermoelectric material according tocomparative example 3. It can be said that a black spot seen in FIG. 14is just a pore, but is not a nano-dot.

Additionally, for comparison of examples, a description is provided withreference to FIGS. 15 and 16 because it is not easy to distinguish theexamples in FIGS. 10 and 11.

FIGS. 15 and 16 are graphs with a change in y-axis scale only for theexamples in FIGS. 10 and 11.

Referring to FIGS. 15 and 16, it can be seen that the thermoelectricmaterial according to the present disclosure represented by the chemicalformula 1 (Cu_(x)Se) has a much lower thermal diffusivity and a muchhigher Seebeck coefficient, when x≦2.04, more specifically, x≦2.05.

Further, seeing the thermal diffusivity (TD) result of FIG. 15, it canbe found that the thermal diffusivity of examples 3 through 7 in which xin chemical formula 1 is higher than 2.04 is generally lower thanexamples 1 and 2 in which x in chemical formula 1 is lower than 2.04.Particularly, examples 5 through 7, more specifically, examples 5 and 6show remarkably low results in the temperature range of 200° C. to 600°C.

Also, seeing the Seebeck coefficient (S) result of FIG. 16, it can befound that examples 3 through 7 in which x in chemical formula 1 ishigher than 2.04 are generally higher in Seebeck coefficient thanexamples 1 and 2 in which x in chemical formula 1 is lower than 2.04.Particularly, for examples 5 through 7, the Seebeck coefficient is foundmuch higher than that of the other examples. Further, in the range of100° C. to 200° C., and in the range of 400° C. to 600° C., the Seebeckcoefficient of examples 6 and 7 is found much higher than that of theother examples.

As described in the foregoing, the thermoelectric material according tothe present disclosure is preferably synthesized by a solid statereaction (SSR) method. Hereinafter, a description of the SSR synthesismethod and its effect is provided in comparison to a melting method.

EXAMPLE 8

Cu and Se in powder form were weighed based on a chemical formulaCu_(2.025)Se, and put in an alumina mortar, followed by mixing. Themixed materials were put into a hard mold, formed into pellets, put in afused silica tube, and vacuum-sealed. Also, the result was put in a boxfurnace, and heated at 1100° C. for 12 hours, and in this instance, atemperature increase time was 9 hours. Then, the result was heated at800° C. for 24 hours again, and in this instance, a temperature decreasetime was 24 hours. After heating, the result was slowly cooled down toroom temperature to obtain a compound.

Also, the compound was filled in a hard mold for hot pressing, and washot press sintered in the condition of 650° C. under vacuum to obtain asample of example 8. In this instance, a sintering density was at least98% of a theoretical value.

EXAMPLE 9

Cu and Se in powder form were weighed based on a chemical formulaCu_(2.1)Se, and mixed and synthesized by the same process as example 8to obtain a compound. Also, the compound was sintered by the sameprocess as example 8 to obtain a sample of example 9.

The samples according to examples 8 and 9 differ in synthesis methodfrom the previous examples 1 through 7. That is, in the case of thesamples according to examples 1 through 7, the thermoelectric materialwas synthesized by an SSR method by which synthesis was performed in astate that at least some of the raw materials did not melt, but in thecase of the samples according to examples 8 and 9, the thermoelectricmaterial was synthesized by a melting method by which all the rawmaterials were heated beyond the melting point.

FIGS. 17 through 19 are graphs illustrating a comparison of measurementresults of a lattice thermal conductivity (K_(L)), a power factor (PF),and a ZT value based on temperature for example 2, example 5, example 8,and example 9.

First, in FIG. 17, the lattice thermal conductivity was calculated usingthe Wiedemann-Franz Law, and in this instance, the Lorenz number usedwas 1.86*10⁻⁸. More specifically, the lattice thermal conductivity maybe calculated using the following mathematical formula:

κ_(L)=κ_(total)−κ_(e)

Here, κ_(L) denotes the lattice thermal conductivity, κ_(total) denotesthe thermal conductivity, and κ_(c) denotes the thermal conductivity tothe electrical conductivity. Also, κ_(e) may be expressed as below:κ_(e)=σLT

Here, σ denotes the electrical conductivity, and L denotes the Lorenznumber and represents 1.86 E-8. Also, T denotes the temperature (K).

Referring to the result of FIG. 17, it can be seen that the latticethermal conductivity of examples 2 and 5 synthesized by an SSR method islower than that of examples 8 and 9 synthesized by a melting method.Particularly, when comparing examples 2 and 8 of the same composition, alattice thermal conductivity change pattern based on temperature issimilar, but in the case of example 2, the lattice thermal conductivityis found remarkably low in the entire temperature range of 100° C. to600° C., compared to example 8. Also, when comparing example 5 andexample 9 of the same composition, the lattice thermal conductivity ofexample 5 by an SSR method is lower than the lattice thermalconductivity of example 9 in the temperature range of 200° C. to 600°C., and moreover, it is found that as the temperature increases, itsdifference increases.

Next, referring to the result of FIG. 18, it can be seen that the powerfactor (PF) of example 2 and example 5 synthesized by an SSR method ishigher than that of example 8 and example 9 synthesized by a meltingmethod. Particularly, when comparing example 2 and example 8 of the samecomposition, example 2 based on an SSR method is found higher in powerfactor than example 8 based on a melting method in the entiretemperature measurement range of 100° C. to 600° C. Also, when comparingexample 5 and example 9 of the same composition, example 5 is foundhigher than example 9 in the entire temperature measurement range of100° C. to 600° C.

Finally, referring to the result of FIG. 19, it can be seen that the ZTof example 2 and example 5 synthesized by an SSR method is higher thanthat of example 8 and example 9 synthesized by a melting method.Particularly, when comparing example 2 and example 8 of the samecomposition, example 2 based on an SSR method is found higher in ZT thanexample 8 based on a melting method in the temperature measurement rangeof 200° C. to 600° C. Also, when comparing example 5 and example 9 ofthe same composition, example 5 is found higher than example 9 in theentire temperature measurement range of 100° C. to 600° C.

Considering this comprehensively, in the case of the thermoelectricmaterial according to the present disclosure, synthesis by an SSR methodmay contribute to higher thermoelectric performance than synthesis by amelting method.

Hereinabove, the present disclosure has been described in detail.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of thedisclosure, are given by way of illustration only, since various changesand modifications within the spirit and scope of the disclosure willbecome apparent to those skilled in the art from this detaileddescription.

What is claimed is:
 1. A thermoelectric material comprising: Cu and Se,wherein the thermoelectric material is represented by the followingchemical formula 1:Cu_(x)Se,  <Chemical Formula 1> wherein 2<x≦2.6 in the chemical formula1, and wherein the thermoelectric material has a plurality of differentcrystal structures together, in which Cu atoms and Se atoms are arrangedin the crystal, at a predetermined temperature of 100° C. to 300° C. 2.The thermoelectric material according to claim 1, wherein a monocliniccrystal structure and a cubic crystal structure are present together at100° C.
 3. The thermoelectric material according to claim 2, wherein onetype of monoclinic crystal structure and two types of cubic crystalstructures are present together at 100° C.
 4. The thermoelectricmaterial according to claim 1, wherein two different cubic crystalstructures of different space groups are present together at one or moretemperatures of 150° C., 200° C., and 250° C.
 5. The thermoelectricmaterial according to claim 4, wherein the different space groups of thecubic crystal structures are indicated by Fm-3m and F-43m, respectively.6. A thermoelectric conversion element comprising a thermoelectricmaterial according to claim
 1. 7. A thermoelectric power generatorcomprising a thermoelectric material according to claim 1.